Vacuum infusion or VARTM (vacuum assisted resin transfer moulding) is one method, which is typically employed for manufacturing composite structures, such as wind turbine blades comprising a fibre reinforced matrix material. During the manufacturing process, liquid polymer, also called resin, is filled into a mould cavity, in which fibre material priorly has been inserted, and where a vacuum is generated in the mould cavity hereby drawing in the polymer. The polymer can be thermoset plastic or thermoplastics. Typically, uniformly distributed fibres are layered in a first rigid mould part, the fibres being rovings, i.e. bundles of fibre bands, bands of rovings or mats, which are either felt mats made of individual fibres or woven mats made of fibre rovings. A second mould part, which is often made of a resilient vacuum bag, is subsequently placed on top of the fibre material and sealed against the first mould part in order to generate a mould cavity. By generating a vacuum, typically 80 to 95% of the total vacuum, in the mould cavity between the inner side of the mould part and the vacuum bag, the liquid polymer can be drawn in and fill the mould cavity with the fibre material contained herein. So-called distribution layers or distribution tubes, also called inlet channels, are used between the vacuum bag and the fibre material in order to obtain as sound and efficient a distribution of polymer as possible. In most cases the polymer applied is polyester or epoxy, and the fibre reinforcement is most often based on glass fibres or carbon fibres.
During the process of filling the mould, a vacuum, said vacuum in this connection being understood as an under-pressure or negative pressure, is generated via vacuum outlets in the mould cavity, whereby liquid polymer is drawn into the mould cavity via the inlet channels in order to fill said mould cavity. From the inlet channels the polymer disperses in all directions in the mould cavity due to the negative pressure as a flow front moves towards the vacuum channels. Thus, it is important to position the inlet channels and vacuum channels optimally in order to obtain a complete filling of the mould cavity. Ensuring a complete distribution of the polymer in the entire mould cavity is, however, often difficult, and accordingly this often results in so-called dry spots, i.e. areas with fibre material not being sufficiently impregnated with resin. Thus dry spots are areas where the fibre material is not impregnated, and where there can be air pockets, which are difficult or impossible to remove by controlling the vacuum pressure and a possible overpressure at the inlet side. In vacuum infusion techniques employing a rigid mould part and a resilient mould part in the form of a vacuum bag, the dry spots can be repaired after the process of filling the mould by puncturing the bag in the respective location and by drawing out air for example by means of a syringe needle. Liquid polymer can optionally be injected in the respective location, and this can for example be done by means of a syringe needle as well. This is a time-consuming and tiresome process. In the case of large mould parts, staff have to stand on the vacuum bag. This is not desirable, especially not when the polymer has not hardened, as it can result in deformations in the inserted fibre material and thus in a local weakening of the structure, which can cause for instance buckling effects.
Often the composite structures comprise a core material covered with a fibre reinforced material, such as one or more fibre reinforced polymer layers. The core material can be used as a spacer between such layers to form a sandwich structure and is typically made of a rigid, lightweight material in order to reduce the weight of the composite structure. In order to ensure an efficient distribution of the liquid resin during the impregnation process, the core material may be provided with a resin distribution network, for instance by providing channels or grooves in the surface of the core material.
As for instance blades for wind turbines have become bigger and bigger in the course of time and may now be more than 60 meters long, the impregnation time in connection with manufacturing such blades has increased, as more fibre material has to be impregnated with polymer. Furthermore the infusion process has become more complicated, as the impregnation of large shell members, such as blades, requires control of the flow fronts to avoid dry spots, said control may e.g. include a time-related control of inlet channels and vacuum channels. This increases the time required for drawing in or injecting polymer. As a result the polymer has to stay liquid for a longer time, normally also resulting in an increase in the curing time.
Resin transfer moulding (RTM) is a manufacturing method, which is similar to VARTM. In RTM the liquid resin is not drawn into the mould cavity due to a vacuum generated in the mould cavity. Instead the liquid resin is forced into the mould cavity via an overpressure at the inlet side.
Prepreg moulding is a method in which reinforcement fibres are pre-impregnated with a pre-catalysed resin. The resin is typically solid or near-solid at room temperature. The prepregs are arranged by hand or machine onto a mould surface, vacuum bagged and then heated to a temperature, where the resin is allowed to reflow and eventually cured. This method has the main advantage that the resin content in the fibre material is accurately set beforehand. The prepregs are easy and clean to work with and make automation and labour saving feasible. The disadvantage with prepregs is that the material cost is higher than for non-impregnated fibres. Further, the core material need to be made of a material, which is able to withstand the process temperatures needed for bringing the resin to reflow. Prepreg moulding may be used both in connection with a RTM and a VARTM process.
Further, it is possible to manufacture hollow mouldings in one piece by use of outer mould parts and a mould core. Such a method is for instance described in EP 1 310 351 and may readily be combined with RTM, VARTM and prepreg moulding.
FR2881371A describes a method for manufacturing small tubular composite structures, such as seat frames, having a diameter of a few centimeters the most. The procedure consists of inserting a component made from braided metal filaments and thermoplastic fibres in a mould cavity and heating the metal by induction to melt the thermoplastic. The heat is then shut off to allow the plastic to cool and set, and the plastic coated component is then removed from the mould. A section of the braided component can be stretched or compressed to reduce or increase its diameter while it is being inserted in the mould, and during the heating and cooling stages it can be held against the walls of the mould cavity by a magnetic field, using electromagnets for braiding of a magnetic material. The braided component can alternatively be made from plastic-coated metal filaments.
A wind turbine blade comprises a number of relatively complex contours or profiles in the longitudinal direction of the blade. Over the years, the shape of conventional wind turbine blades has developed towards a design comprising a root region with a substantially circular or elliptical profile closest to the hub, and an airfoil region with a lift generating profile furthest away from the hub. The blade optionally comprises a transition region between the root region and the airfoil region, wherein the profile of the transition region gradually changes in the radial direction from the circular profile of the root region to the lift generating profile of the airfoil region. Typically, the airfoil region extends from a position of a maximum chord length to the tip end of the blade. This position is typically located at a radial distance from the root of about 20% of the blade length. The suction side of the blade typically has a convex envelope, whereas the pressure side for instance may comprise a double curvature, i.e. partly a convex envelope and partly a concave curvature. Consequently, the mould parts need to have a similar complex structure.
During the manufacturing of such wind turbine blades a number of fibre layers are arranged above the forming surface of the mould part. Due to varying curvature of the forming surface retaining means, such as clamping means, are often used in order to retain or secure the fibre layers against the forming surface of the mould. This is especially apparent at the root region of the blade due to the circular profile. However, the clamping means need to be loosened every time a new layer of fibres is arranged above a previous layer. Subsequently, the clamping means need to be reclamped in order to retain the fibre layers against the forming surface. This is a tedious process and in worst case the clamping means may damage the fibre layers, causing local weaknesses in the finished wind turbine blade.